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Support for multiple provider credentials

Currently, Velero only supports a single credential secret per location provider/plugin. Velero creates and stores the plugin credential secret under the hard-coded key secret.cloud-credentials.data.cloud.

This makes it so switching from one plugin to another necessitates overriding the existing credential secret with the appropriate one for the new plugin.

Goals

To allow Velero to create and store multiple secrets for provider credentials, even multiple credentials for the same provider
To improve the UX for configuring the velero deployment with multiple plugins/providers.
Enable use cases such as AWS volume snapshots w/Minio as the object storage
Continue to support use cases where multiple Backup Storage Locations are in use simultaneously
- velero backup logs while backup/restore is running
Handle changes in configuration while operations are happening as well as they currently are

Non Goals

To make any change except what's necessary to handle multiple credentials
To allow multiple credentials for or change the UX for node-based authentication (e.g. AWS IAM, GCP Workload Identity, Azure AAD Pod Identity). Node-based authentication will not allow cases such as a mix of AWS snapshots with Minio object storage.

Design overview

Instead of one credential per Velero deployment, multiple credentials can be added and used with different BSLs.

There are two aspects to handling multiple credentials:

Modifying how credentials are configured and specified by the user
Modifying how credentials are provided to the plugin processes

Each of these aspects will be discussed in turn.

Credential configuration

Currently, Velero creates a secret (cloud-credentials) during install with a single entry that contains the contents of the credentials file passed by the user.

Instead of adding new CLI options to Velero to create and manage credentials, users will create their own Kubernetes secrets within the Velero namespace and reference these. This approach is being chosen as it allows users to directly manage Kubernetes secrets objects as they wish and it removes the need for wrapper functions to be created within Velero to manage the creation of secrets. Separate credentials rather than combining credentials in a single secret also avoids issues with maximum size of credentials as well as update in place issues.

To enable the use of existing Kubernetes secrets, BSLs will be modified to have a new field Credential. This field will be a SecretKeySelector which will enable the user to specify which key within a particular secret the BSL should use.

Existing BackupStorageLocationSpec definition:

// BackupStorageLocationSpec defines the desired state of a Velero BackupStorageLocation
type BackupStorageLocationSpec struct {
	// Provider is the provider of the backup storage.
	Provider string `json:"provider"`

	// Config is for provider-specific configuration fields.
	// +optional
	Config map[string]string `json:"config,omitempty"`

	StorageType `json:",inline"`

	// Default indicates this location is the default backup storage location.
	// +optional
	Default bool `json:"default,omitempty"`

	// AccessMode defines the permissions for the backup storage location.
	// +optional
	AccessMode BackupStorageLocationAccessMode `json:"accessMode,omitempty"`

	// BackupSyncPeriod defines how frequently to sync backup API objects from object storage. A value of 0 disables sync.
	// +optional
	// +nullable
	BackupSyncPeriod *metav1.Duration `json:"backupSyncPeriod,omitempty"`

	// ValidationFrequency defines how frequently to validate the corresponding object storage. A value of 0 disables validation.
	// +optional
	// +nullable
	ValidationFrequency *metav1.Duration `json:"validationFrequency,omitempty"`
}

The following field will be added:

Credential *corev1api.SecretKeySelector `json:"credential,omitempty"`

The resulting BackupStorageLocationSpec will be this:

// BackupStorageLocationSpec defines the desired state of a Velero BackupStorageLocation
type BackupStorageLocationSpec struct {
	// Provider is the provider of the backup storage.
	Provider string `json:"provider"`

	// Config is for provider-specific configuration fields.
	// +optional
	Config map[string]string `json:"config,omitempty"`

	// Credential contains the credential information intended to be used with this location
	// +optional
	Credential *corev1api.SecretKeySelector `json:"credential,omitempty"`

	StorageType `json:",inline"`

	// Default indicates this location is the default backup storage location.
	// +optional
	Default bool `json:"default,omitempty"`

	// AccessMode defines the permissions for the backup storage location.
	// +optional
	AccessMode BackupStorageLocationAccessMode `json:"accessMode,omitempty"`

	// BackupSyncPeriod defines how frequently to sync backup API objects from object storage. A value of 0 disables sync.
	// +optional
	// +nullable
	BackupSyncPeriod *metav1.Duration `json:"backupSyncPeriod,omitempty"`

	// ValidationFrequency defines how frequently to validate the corresponding object storage. A value of 0 disables validation.
	// +optional
	// +nullable
	ValidationFrequency *metav1.Duration `json:"validationFrequency,omitempty"`
}

The CLI for managing Backup Storage Locations (BSLs) will be modified to allow the user to set these credentials. Both velero backup-location (create|set) will have a new flag (--credential) to specify the secret and key within the secret to use. This flag will take a key-value pair in the format <secret-name>=<key-in-secret>. The arguments will be validated to ensure that the secret exists in the Velero namespace. If the Credential field is empty in a BSL, the default credentials from cloud-credentials will be used as they are currently.

Making credentials available to plugins

The approach we have chosen is to include the path to the credentials file in the config map passed to a plugin.

Including the credentials file path in the `config` map

Prior to using any secret for a BSL, it will need to be serialized to disk. Using the details in the Credential field in the BSL, the contents of the Secret will be read and serialized. To achieve this, we will create a new package, credentials, which will introduce new types and functions to manage the fetching of credentials based on a SecretKeySelector. This will also be responsible for serializing the fetched credentials to a temporary directory on the Velero pod filesystem.

The path where a set of credentials will be written to will be a fixed path based on the namespace, name, and key from the secret rather than a randomly named file as is usual with temporary files. The reason for this is that BackupStores are frequently created within the controllers and the credentials must be serialized before any plugin APIs are called, which would result in a quick accumulation of temporary credentials files. For example, the default validation frequency for BackupStorageLocations is one minute. This means that any time a BackupStore, or other type which requires credentials, is created, the credentials will be fetched from the API server and may overwrite any existing use of that credential.

If we instead wanted to use an unique file each time, we could work around the of multiple files being written by cleaning up the temporary files upon completion of the plugin operations, if this information is known.

Once the credentials have been serialized, this path will be made available to the plugins. Instead of setting the necessary environment variable for the plugin process, the config map for the BSL will be modified to include an addiitional entry with the path to the credentials file: credentialsFile. This will be passed through when initializing the BSL and it will be the responsibility of the plugin to use the passed credentials when starting a session. For an example of how this would affect the AWS plugin, see this PR.

The restic controllers will also need to be updated to use the correct credentials. The BackupStorageLocation for a given PVB/PVR will be fetched and the Credential field from that BSL will be serialized. The existing setup for the restic commands use the credentials from the environment variables with some repo provider specific overrides. Instead of relying on the existing environment variables, if there are credentials for a particular BSL, the environment will be specifically created for each RepoIdentifier. This will use a lot of the existing logic with the exception that it will be modified to work with a serialized secret rather than find the secret file from an environment variable. Currently, GCP is the only provider that relies on the existing environment variables with no specific overrides. For GCP, the environment variable will be overwritten with the path of the serialized secret.

Split credentials between VolumeSnapshotter and ObjectStore plugins

One of the use cases we wish to satisfy is the ability to specify a different object store than the cloud provider offers, for example, using a Minio S3 object store from within AWS. Currently the VolumeSnapshotter and the ObjectStore plugin share the cloud credentials. Each backup/restore has a BackupStorageLocation associated with it. The BackupStorageLocation can optionally specify the credential used by the ObjectStorePlugin and Restic daemons while the cloud credential will always be used for the VolumeSnapshotter.

Velero Plugin for vSphere compatibility

The vSphere plugin is implemented as a BackupItemAction and shares the credentials of the AWS plugin for S3 access. The backup storage location is passed in Backup.Spec.StorageLocation. Currently the plugin retrieves the S3 bucket and server from the BSL and creates a BackupRespositoryClaim with that and the credentials retrieved from the cloud credential. The plugin will need to be modified to retrieve the credentials field from the BSL and use that credential in the BackupRepositoryClaim.

Backwards compatibility

For now, regardless of the approaches used above, we will still support the existing workflow.

Users will be able to set credentials during install and a secret will be created for them. This secret will still be mounted into the Velero pods and the appropriate environment variables set. This will allow users to use versions of plugins which haven't yet been updated to use credentials directly, such as with many community created plugins.

Multiple credential handling will only be used in the case where a particular BSL has been modified to use an existing secret.

Security Considerations

Although the handling of secrets will be similar to how credentials are currently managed within Velero, care must be taken to ensure that any new code does not leak the contents of secrets, for example, including them within logs.

Parallelism

In order to support parallelism, Velero will need to be able to use multiple credentials simultaneously with the ObjectStore. Currently backups are single threaded and a single BSL will be used throughout the entire backup. The only existing points of parallelism are when a user downloads logs for a backup or the BackupStorageLocationReconciler reconciles while a backup or restore is running. In the current code, download_request_controller.go and backup_storage_location_controller.go create a new plugin manager and hence another ObjectStore plugin in parallel with the ObjectStore plugin servicing a backup or restore (if one is running).

Alternatives Considered

Three different approaches can be taken to provide credentials to plugin processes:

Providing the path to the credentials file as an environment variable per plugin. This is how credentials are currently passed.
Include the path to the credentials file in the config map passed to a plugin.
Include the details of the secret in the config map passed to a plugin.

The last two options require changes to the plugin as the plugin will need to instantiate a client using the provided credentials. The client libraries used by the plugins will not be able to rely on the credentials details being available in the environment as they currently do.

We have selected option 2 as the approach to take.

The approaches that were not selected are detailed below for reference.

Providing the credentials via environment variables

To continue to provide the credentials via the environment, plugins will need to be invoked differently so that the correct credential is used. Currently, there is a single secret, which is mounted into every pod deployed by Velero (the Velero Deployment and the Restic DaemonSet) at the path /credentials/cloud. This path is made known to all plugins through provider specific environment variables and all possible provider environment variables are set to this path.

Instead of setting the environment variables for all the pods, we can modify plugin processes are created so that the environment variables are set on a per plugin process basis. Prior to using any secret for a BSL, it will need to be serialized to disk. Using the details in the Credential field in the BSL, the contents of the Secret will be read and serialized to a file. Each plugin process would still have the same set of environment variables set, however the value used for each of these variables would instead be the path to the serialized secret.

To set the environment variables for a plugin process, the plugin manager must be modified so that when creating an ObjectStore, we pass in the entire BSL object, rather than just the provider. The plugin manager currently stores a map of plugin executables to an associated RestartableProcess. New restartable processes are created only with the executable that the process would run. This could be modified to also take the necessary environment variables so that when underlying go-plugin process is created, these environment variables could be provided and would be set on the plugin process.

Taking this approach would not require any changes from plugins as the credentials information would be made available to them in the same way. However, it is quite a significant change in how we initialize and invoke plugins.

We would also need to ensure that the restic controllers are updated in the same way so that correct credentials are used (when creating a ResticRepository or processing PodVolumeBackup/PodVolumeRestore). This could be achieved by modifying the existing function to run a restic command. This function already sets environment variables for the restic process depending on which storage provider is being used.

Include the details of the secret in `config` map passed to a plugin

This approach is like the selected approach of passing the credentials file via the config map, however instead of the Velero process being responsible for serializing the file to disk prior to invoking the plugin, the Credential SecretKeySelector details will be passed through to the plugin. It will be the responsibility of the plugin to fetch the secret from the Kubernetes API and perform the necessary steps to make it available for use when creating a session, for example, serializing the contents to disk, or evaluating the contents and adding to the process environment.

This approach has an additional burden on the plugin author over the previous approach as it requires the author to create a client to communicate with the Kubernetes API to retrieve the secret. Although it would be the responsibility of the plugin to serialize the credential and use it directly, Velero would still be responsible for serializing the secret so that it could be used with the restic controllers as in the selected approach.

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